“…There are many possible ways to study the Hele-Shaw equation: to mention a few approaches we quote various PDE methods based on L 2 -energy estimates (see the works of Chen [15], Córdoba, Córdoba and Gancedo [21], Knüpfer and Masmoudi [32], Günther and Prokert [29], Cheng, Granero-Belinchón and Shkoller [16]), there are also methods based on functional analysis tools and maximal estimates (see Escher and Simonett [26], the results reviewed in the book by Prüss and Simonett [38] and Matioc [34,35]) or methods using harmonic analysis tools and contour integrals (see the numerous results reviewed in the survey papers by Gancedo [27] or Granero-Belinchón and Lazar [28]). For the related Muskat equation (a two-phase Hele-Shaw problem), maximum principles have played a key role to study the Cauchy problem, see [12,18,10,22] following the pioneering work of Constantin, Córdoba, Gancedo, Rodríguez-Piazza and Strain [17].…”